The invention relates to a fastening assembly for fastening a fixing element, for example a plate, brace, angle, profile, armature, retainer or the like, to a substructure by means of at least one screw which can be passed through at least one bore in the fixing element and can be screwed into the substructure.
Numerous fastening assemblies of the mentioned type are known, which are provided for many different types of applications.
For example, such fastening assemblies are used in the house-hold, for example to mount objects on the wall or ceiling, such as lamps, shelves or the like. Normally, fixing elements in the form of wall fixtures, profiles or the like are provided for such objects which must then be secured to the wall or the ceiling by means of one or more screws.
Such fixing elements normally have one or more bores through which a screw is passed and screwed into the substructure. In such cases, a hole is first drilled into the wall or the ceiling into which a dowel is inserted and the screw is then screwed into the dowel. By completely and firmly tightening the screw, the screw head forces the fixing element against the substructure.
Due to inexperience of the user in household applications or due to improper tools, it frequently occurs that the hole in the wall or the ceiling is not perpendicular to the surface, but is drilled at an angle. In particular, it is difficult to drill a perpendicular hole in the wall or ceiling when located close to a corner of the room, because the housing of the drilling machine makes it difficult or impossible to place the drill perpendicular to the ceiling or the wall.
If the hole in the wall or the ceiling is slanted, the screw must necessarily be screwed in at an angle through the bore in the fixing element and into the substructure. The shaft of the screw normally has a diameter which is only slightly smaller than the bore so that the shaft of the screw can clamp with the upper and/or lower edge of the bore. The screw can then only be applied with increased force exertion. In the worst case, it is not even possible to completely tighten down the screw and the fixing element is then not securely and immovably fixed to the substructure. This drawback can also not be avoided by an elongated configuration of the bore.
Even if the screw can be completely tightened and drawn down to the substructure, the screw head only contacts the edge of the bore. In the case of a countersunk screw, in some cases only a point contact is made with the fixing element. This does not ensure a solid play-free securement of the fixing element to the substructure.
Another application of such fastening assemblies is the do-it-yourself assembly of furniture, for example mounting hinges on cabinet doors or the like. In such cases, self-cutting wood screws are used as the screws. When mounting an angle, it is also frequently difficult to turn down the wood screws to be perpendicular to the cabinet wall because of the difficult access in the region of the angle. As soon as a few windings of the screw engage with the substructure, the orientation of the shaft with respect to the substructure and therefore with respect to the fixing element normally can not be corrected. The same problem then arises that the screws can not be completely tightened or only with excessive force to secure the fixing element immovably to the substructure.
A further important application of the mentioned fastening assembly lies in the medical field. Braces or splints are implanted in patients to heal bone fractures by fixing the broken bone portions, which are formed as elongated metal plates. At least two bores are provided in the brace, so that the brace can be applied across the fracture connecting the two bone portions. Bone screws are used as the screws, which are directly applied to the bones. Due the anatomical conditions of the patient or the location of bone fracture, it is also often necessary to apply the bone screws at an angle with respect to the bone brace. The same problem arises here as described above in conjunction with the technical applications.
In addition, for this application the bone parts to which the brace is to be secured do not have a flat uniform surface but have concave and convex undulations. In order to fix the brace immovably to the bone parts, the bone screw can not be screwed into the bone with excessive force. This would cause undesired tension or could even lead to an additional fracture of the bone. It is therefore necessary that the bone screws establish a rigid connection of the brace with the bone parts without having to exert an excessive force, through which the bone parts would otherwise be pressed onto the brace and placed under stress.
A fastening assembly allowing a screw to be screwed into the substructure at an angle with respect to the fixing element without increased force exertion and allowing an immovable connection free of play is disclosed in U.S. Pat. No. 4,388,921. This known fastening assembly comprises a bushing arranged in the bore of the fixing element, through which the screw can be passed for screwing the screw into the substructure. The bushing of this known fastening assembly is able to swivel in several spacial directions when seated in the bore. The bushing further comprises a seat for receiving the head of the screw. An inner surface of the bore and an outer surface of the bushing are formed to be about spherically curved, wherein a region of largest diameter of the inner surface of the bore is situated at the upper edge of the bore. By that the bushing can fall out of the bore as long as the screw is not screwed into the sub-structure. Therefore, the handling of this known fastening assembly is somewhat cumbersome because there are three parts, namely the fixing element, the screw and the bushing which must be held by hand when using the fastening assembly. In particular, in case that the fastening assembly must be fastened under a ceiling the bushing can fall out when manipulating the fixing elements and the screw. In medical applications there exists the risk that the bushing is lost in the human body.
The object of the present invention is therefore to provide a fastening assembly which is improved in terms of its handling properties.